Various methods are known for detecting leaks. For example, to detect leaks at junctions between tubes or other hollow members, the region about the junction is commonly isolated to form a test chamber which is filed with a test gas under pressure. The pressure of the compressed gas is then monitored, a drop in pressure indicating a leak. In another test procedure for two interconnected tubular members, a test chamber within the members is formed by means of a shut-off tool and gas pressure is applied to the test chamber. A process of this kind and a tool for use in the process is described in German OS No. 30 30 665. In this process two spaced-apart annular seals of resilient material are axially compressed by respective pistons. This urges the seals radially outwardly hard against the walls of the interconnected tubular members, thereby forming a test chamber into which helium or other test gas is introduced under pressure. Instruments for detecting escaping test gas are connected to the outside of the tubular members circumjacent the test chamber.
The above test processes and apparatus have the disadvantage of requiring large quantities of test gas which has to be compressed to very high pressures. The combination of high pressure and large amounts of the test gas creates a significant hazard not only adjacent the test zone but also adjacent the source of the compressed gas and along the supply line extending between said source and the test zone. Furthermore, existing shut-off tools are only suitable for pipes and other elongate cavities that have a substantially uniform diameter. They are unsuitable for testing the gas-tightness of non-cylindrical or irregularly shaped structures such as tanks. Moreover, the danger inherent in a compressed gas leak test very significantly increases as the size of the tank to be tested increases. The normal response to this danger is to use a very low test pressure, but that in turn considerably reduces the value of the test.